First off you always start with the batteries. They need to be charged and tested with a hydrometer for one or more dead cells. That's the most common reason for this problem. Secondly, if any part of your alternator was going bad and it was draining your batteries, as the batteries got lower the electrical system would let you know. Your ABS light will generally be the first light to pop on, followed by your amp light (looks like a battery), and your lights would dim, etc. If you then "fully" charged the batteries the vehicle would run fine until the batteries were low again. Alternators do fail in high heat fairly often, and new alts go bad all the time especially from places like autozone, but thats not the only thing that can drain your batteries. First you have to make sure your getting proper power at the alt output (12v) and power at the regulator plug (4-12v generally). If you don't it doesn't matter how many new alts you put on. If your alt. is not the problem you may have a drain in your electrical system. Which could be anything from an oversized stereo system to a short in a wire. To test for this buy a 12v test light, with the vehicle off, disconnect the negative terminal from the battery, connect one end of the test light to the terminal on the battery and the other end to the terminal on the car. Make sure everything, as far as you can tell, in the vehicle is off. If the tester lights up you have a parasitic drain. Some drains are normal, but excessive drains can kill a bettery in a day or two. If the light is dim it usually is normal drain. If it's blinking it usually is a security system (normal). If it's very bright, then it's at least a decent size drain. Also expensive batteries are not always better batteries. You should generally spend between $50-60.00 on a battery around 500-600cca. Higher cca batteries tend to be more unpredictable. | 
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